What is vector-borne disease?

  • A micro- or macroparasite transmitted among vertebrate hosts by an arthropod vector

  • Vector-borne diseases represent some of the deadliest, most impoverishing diseases of mankind and wildlife

Vectors

Vectors

Importance of vector-borne disease

  • Vectors are flying syringes

  • Host preference determines encounter

  • Huge economic cost

  • Some rough diseases

Vectors are flying syringes

  • Vectors take blood or host tissue (including pathogen) and transmit to same (or different) species

  • Means reservoir hosts are really important

Host preference determines encounter

  • Vectors prefer certain hosts, and host preference determines the resulting disease burden

Eigenbrode & Gomulkiewicz 2022 J Economic Entomology

Huge economic cost

  • $12 billion per year (Chilakam et al. 2023 doi:10.2196/50985)

  • $100 billion economic costs per year (USDA-ARS report)

  • More important is the cost in terms of impacts to human and wildlife

Should we ‘extinct’ some species?

“Keep their (Anopheles gambiae) DNA for future research and let them go” -EO Wilson

Fang 2010 Nature; and associated 4+ replies

Some rough diseases

Mosquitos

  • Malaria, Dengue, West Nile, Chikv, Zika, yellow fever,

Ticks:

  • Lyme, E equine encephalitis, Ehrlichiosis, Rocky Mtn spotted fever, Tularemia

Other:

  • Plague (fleas), Chagas (Tiatome bugs), Schisto (aquatic snails), Sleeping sickness (Tsetse flies)

Plague

  • Yersinia pestis, a bacterial pathogen

  • Vectored by fleas

  • Killed 25% of Europe’s pop in 1300’s

  • Exists in enzootic cycles with transmission between rodents and fleas

Lyme disease

  • Borrelia burgdorferi, a bacterial pathogen

  • Vectored by ticks (Ixodes scapularis as main vector)

  • Ancient disease (plagued humans prior to European Colonization)

  • Disappeared around 1850’s due to deforestation

Lyme disease

Mosquito-borne disease is different

  • Dengue (33%+ of world pop at risk)

  • Chikungunya (45 countries affected)

  • Zika (61 countries affected, 47 in Americas)

  • Malaria (kills ~1 million people per year)

Mosquito-borne disease is different

Flores & O’Neill 2018 Nature Reviews Microbiology

And we should be worried

Ainsworth 2023 Nature

How do we model vector-borne disease?

  • Recall the SIR model

Ross-MacDonald model

Ross-MacDonald model

Ross-MacDonald model

  • Host and vector terms are both in there

  • We can look across probable parameter ranges to see the effect of each component

  • Is it more important to reduce vector infection probability ( \(b_v\) ) or host duration of infection ( \(1/r\) )

Ross-MacDonald model

Ross-MacDonald model

  • Controlling mosquitoes has largest effect

Let’s kill the mosquitoes

  • \(R_0\) is most sensitive to changes in mosquito survival

Let’s not kill the mosquitoes (or at least find better ways)

Hancock et al. 2018 PNAS

What about the other terms in \(R_0\)?

What about the other terms in \(R_0\)?




End of lecture 1

What have we learned?

  • Vector-borne disease is pretty rough

  • We have a decent model of vector-borne disease

  • It can help identify suitable mitigation strategies

The role of environment on vector-borne disease

The role of environment on vector-borne disease

The role of environment on vector-borne disease

Does this mean climate change is ‘good’?

  • Mixed, but probably not

The role of vector preference on resulting disease dynamics

The role of vector preference on resulting disease dynamics

The role of vector preference on resulting disease dynamics

Important because:

  • Vector feeding preferences are particularly important for multi-host systems

  • Can introduce variation in exposure across different groups of individuals within a host population

  • Vector preference can inform intervention strategies based on vector and reservoir communities

But let’s not be silly about it

But let’s not be silly about it

  • Response to this (and other bat-vectored disease) is to try to kill bats

  • Often killed the wrong bats

  • Some efforts potentially increased disease in humans

The role of pathogen infection on vector behavior

The role of pathogen infection on vector behavior

The role of pathogen infection on vector behavior

The role of pathogen infection on vector behavior

Infected malaria mosquitoes prefer warmer environments

An example of host manipulation by the a pathogen?

The role of vector behavior in response to interventions

The role of vector behavior in response to interventions

The role of vector behavior in response to interventions